This invention relates to voltage regulator output power transistor safe operating area protection (SOA).
A wide variety of protection techniques are known in the prior art, including techniques which reduce output transistor conduction by reducing base (or gate) drive to synthesize current limits, overvoltage limits, and junction temperature limits. What the continuous safe area limit techniques (as opposed to the more simple digital on/off protection techniques) have in common is a resistor that senses the supply voltage for the current limit circuit. However, when the quiescent current of the circuit must be made very low, the required resistor becomes an inefficient solution. For example, a 3 M.OMEGA. resistor sensing a 30 supply draws 10 .mu.A. (A resistor having much higher resistance will consume excessive integrated circuit die area). This amount of current, plus the current consumed by the remainder of the current limiting circuitry, is unacceptably high.
FIG. 1 is a schematic diagram of a typical prior art current source, utilizing resistor 11 to sense supply voltage V.sub.in. The circuit of Figure
FIG. 4 is a schematic diagram of a typical prior art multiplying power limit circuit 200. Circuit 200 includes differential amplifier 230 which provides a control signal to the base of output transistor 205, which in turn controls output voltage V.sub.out. The output current available on output lead 203 is controlled by differential amplifier 230, in response to input voltage V.sub.in applied to lead 201. Transistor 206 is connected in parallel with output transistor 205 in order to mirror the current flowing through output transistor 205. PNP transistor 207 is connected as a load device having one of its collectors 207-1 connected to its base and in turn connected to the collector of transistor 206. Collector 207-2 of transistor 207 provides a current which mirrors the current flowing through collector 207-1 of transistor 207. This mirrored current is applied to load device 208, which in turn causes current source 209 to mirror the current flowing through transistor 208. Current source 209 feeds differential transistor pair 213 and 214, with transistor 213 receiving its base drive input signal from output lead 203. A resistive voltage divider formed of resistors 210 and 211 (having resistances R.sub.2 and R.sub.1, respectively) is connected between V.sub.in lead 201 and V.sub.out lead 203, and provides a voltage to the base of differential transistor 214. Transistor 215 serves as a load device connected between V.sub.in lead 201 and the collector of transistor 213. Transistor 216 mirrors the current flowing through transistor load device 215, and has its collector connected to the inverting input lead of differential amplifier 230 and to the collector of differential input transistor 214. The non-inverting input lead of differential amplifier 230 is connected to a bias voltage above ground provided by bias circuitry 204.
A negative feedback loop includes amplifier 230, pass transistor 205, and the transconductance amplifier consisting of transistors 209, 213, 214, 215, and 216. Because of the high loop gain, the voltages on the inverting and non-inverting input leads of amplifier 230 are forced to be equal. The voltage drop across resistor 212 (having resistance R.sub.3) is therefore equal to the offset voltage V.sub.offset depicted as 231.
If I.sub.x is the output current of the transconductance amplifier then ##EQU1## Eq. (2) thus becomes ##EQU2## Setting this equal to Eq. (1) we get ##EQU3##
Since I.sub.out (V.sub.in -V.sub.out) equals the power dissipation of pass transistor 205, the circuit of FIG. 4 gives a constant power dissipation.
Unfortunately, prior art circuit 200 of FIG. 4 is undesirably sensitive to manufacturing tolerances, for example the inherent DC offset voltage V.sub.offset of differential amplifier 230. Furthermore, prior art circuit 200 requires large value resistors 210 and 211 connected between V.sub.in lead 201 and V.sub.out lead 203, which are difficult to fabricate on an integrated circuit to precise tolerances, and require a large amount of integrated circuit area.
It is an object of this invention to provide a current source whose output falls as V.sub.in -V.sub.out rises.
It is a further object of the invention to provide a current source whose output current may vary as a wide range of functions of V.sub.in -V.sub.out
It is still a further object of the invention to produce the output current as a function of V.sub.in -V.sub.out with any absolute value of current, supply current, and without large valued resistors.